1. Field of the Invention
This invention relates to a telecommunications network and in particular to such a network including back-up circuits. In a telecommunications network a level of redundancy is often provided so that in the event of a circuit failure a back-up route may be used. This invention relates to a telecommunications network in which automatic switching of telecommunications traffic to backup circuits is provided when faults are detected on a traffic carrying primary circuit. In particular, when dedicated private circuits develop faults, this invention provides a network in which automatic switching of traffic to a dedicated back-up circuit takes place. This invention also relates to methods of automatically switching traffic to a back-up route in the event of a circuit failure.
2. Description of Related Art
In a network requiring protection using back-up circuits, each circuit is monitored within an exchange using a conventional test apparatus, sometimes referred to as an In Service Monitoring Network Element (ISMNE), which is connected to various monitor points on a circuit. Each test apparatus generates an alarm when certain error conditions are detected. Each test apparatus is monitored centrally by a Transmission Network Surveillance system (TNS). When alarms indicate that a traffic carrying circuit has failed, a central restoration co-ordinator determines that errors detected by two test apparatus correspond to a particular traffic bearing circuit and the traffic is switched onto a back-up circuit.
The problem with using such a centralized restoration process is that it can take some time for the alarms to be sent to the centralized processor, the alarms to be interpreted and commands to be sent to the exchanges to switch traffic to a backup circuit. Clearly, once a circuit has failed, a customer using that circuit has no service until traffic is switched to a back-up circuit. If a technique can be used which allows each exchange to autonomously switch traffic from a failed primary circuit to a back-up circuit, then the time for which the customer loses service can be considerably reduced.